This invention relates to a method of joining or attaching two surfaces or layers of material and more particularly to a method of welding two surfaces or layers of material in an efficient, time saving and effective manner.
There are known in the prior art a variety of different ways and procedures for joining two layers of material such as by welding. Sometimes this is accomplished by a procedure called spot welding. In spot welding there are at least two layers of material in face-to-face relation. The layers are made of weldable material. At selected points there are applications of electrical current and pressure to cause welding of the layers at the selected points.
In conventional spot welding two layers of material are placed in face-to-face relationship. Electrodes are placed on opposite sides of the layers in alignment with one another and electrical current passed through the electrodes. This technique has several disadvantages which can limit its effectiveness. For example, where a series of spot welds are used to join together layers of material, the current will be split. In other words, a certain amount of current will go directly between the electrodes while some of the current will be shunted to adjacent spot welds. This produces a random variable amount of current available for the new weld depending upon the relative resistance of previously made adjacent welds and the contact resistance of the electrode and layers of material at the point where the new weld will be formed.
Where the layers of material or at least one layer is either non-weldable or difficult to weld a technique sometimes referred to as plug welding is used.
In the plug welding technique, a plug which may be cylindrical or even washer shaped, is formed of weldable material. An opening or hole is formed in the upper layer of material which is not necessarily weldable and the plug placed therein so that it is in contact with the lower layer of material. The problems of current shunting can also occur with this technique and in some techniques using a dielectric coating.
Another technique for joining spaced apart weldable materials involves forming a sandwich like construction. The outer layers of the sandwich must, in this case, be formed of weldable materials. The inner part of the sandwich consists of a dielectric sheet or plate such as Teflon and has weldable balls to join the outer layers of the sandwich. This method of attachment is effective where the layers to be joined are in spaced relationship and where both layers of material are made of weldable material.